Field of the Invention
The present invention relates generally to spacecraft attitude determination and control systems and particularly to on-orbit calibration of gyroscope scale factors.
Brief Description of the Related Art
Gyroscopes (hereinafter, “gyros”) provide measurements of incremental attitude changes or angular rates of a spacecraft. These measurements may be used as part of the attitude determination and control system (hereinafter, “ACS”), and they may be used in conjunction with the active pointing of payload instruments.
Gyros do not measure attitude, but changes in attitude; therefore, an ACS typically also includes one or more sensors capable of measuring attitude, such as star trackers. Star trackers have good performance in low-frequency ranges, but used alone, they may not be suitable for high-bandwidth, precision applications, due to limited output rate and high-frequency noise. Gyros, on the other hand, generally have good high-frequency performance, but their measurements wander or drift over long periods of time. Consequently, gyros alone cannot maintain accurate absolute attitude knowledge and control.
A typical ACS exploits the strengths of both of these sensors, using gyros to propagate an attitude estimate and periodically correcting the estimate with star tracker information. These functions are typically performed by a Kalman Filter having a state vector of six elements, three attitude corrections and three gyro bias corrections. Gyro biases are quasistatic offsets in the angular rate measured by gyros. Since these biases drift over time, the ACS compensates the gyro rate data by subtracting the current bias estimates from the measured rates. The compensated gyro rates are used for propagating the attitude estimate and sometimes for providing control signals to instruments that have an active line of sight control capability.
Gyro errors contributing to vehicle and/or instrument pointing errors include bias, misalignments, and scale factor errors. For nadir-pointing missions and inertial-pointing missions, misalignment and scale factor errors may have negligible impact to steady-state performance or may be indistinguishable from bias errors. In those cases, a dynamic estimate of apparent gyro bias is often sufficient to achieve required performance.
Some missions require gyro scale factor to be accurately known in order to meet requirements. Characteristics of such missions can include a) performing spacecraft slews on gyros only, i.e., under conditions that preclude use of the star trackers during slews, b) stabilizing an instrument line of sight in the presence of dynamic attitude disturbances, or c) maintaining attitude knowledge in the presence of dynamic disturbances. An example is a spacecraft that includes a high-resolution imaging sensor that operates during attitude transients induced by thruster firings or other disturbance events and requires precise line of sight control or precise geolocation of the acquired image data.
Gyro scale factor is typically measured to within the required accuracy prior to spacecraft launch and again during on-orbit spacecraft commissioning. However, gyro scale factor drifts over time, which can result in non-compliant ACS performance. A method to calibrate gyro scale factor over the life of a mission is often required in order to meet requirements.
A prior art method for on-orbit calibration of gyro scale factor includes adding scale factor states to a standard six state onboard Kalman Filter. For mission profiles that include spacecraft slews that are large enough and frequent enough, that is a viable approach. For missions with attitude profiles that are mostly steady-state, e.g., being inertially fixed or rotating at one revolution per orbit, gyro scale factor errors may not be observable enough for a Kalman Filter to accurately estimate them.
Another prior art method overcomes this limitation by executing large-angle spacecraft slews for gyro calibration, making it possible to distinguish the effects of gyro scale factor from gyro bias. Under that approach, data acquired during large calibration slews are processed by an on-board Kalman Filter or on the ground by customary estimation techniques. This method requires periodically taking the spacecraft offline, temporarily suspending normal operations, such as imaging, while performing calibration maneuvers. Some missions cannot accept such a loss in operational availability. Thus, for certain missions, prior art gyro calibration techniques force a choice between system downtime or out-of-specification performance.